1. Field of the Invention
The present invention relates to an image forming apparatus, for example, a laser printer.
2. Description of Prior Art
Recently, the use of optical printers that employ a laser beam of a light-emitting element, e.g., LED, has been spreading. As recording densities of images output by these printers, 240, 300, 400, 480 and 600 dpi (dots/inch) are available in general. These image recording densities are used exclusively for each type of printer and for each printing purpose. However, since it is inconvenient and uneconomical that only one kind of recording density can be obtained with a single printer, there has been a demand for a printer which provides a plurality of recording densities. To meet the demand, printers having two or more recording densities which can be changed for each particular use have recently been developed. One example of this type of printer has control means for changing the laser beam diameter, the synchronizing frequency, the number of revolutions of a polygonal mirror and the developing bias, and enables these factors to be changed by a control operation conducted from an engine control board (see Japanese Patent Laid-Open (KOKAI) No. 01-264850, 1989). Another example of the above-described printers has a plurality of laser sources, and is arranged such that, in response to change-over of recording densities from one to another, the number of revolutions of a polygonal mirror is changed in a relatively low speed range and also the number of laser sources is changed, thereby minimizing the number of revolutions of the polygonal mirror, and thus avoiding difficulties in design that accompany the high speed rotation of the polygonal mirror (see Japanese Patent Laid-Open (KOKAI) No. 01-299042, 1989).
The first example of the prior art has the disadvantage, however, that the control means for changing over image recording densities is complicated, resulting in an increase in the cost. The second example also has the disadvantage that it is necessary not only to change the number of revolutions of the polygonal mirror but also to employ a plurality of light sources. Consequently, the control means is more complicated, resulting in an increase in the cost. In each of the prior art devices, it is necessary, in order to obtain high-density printing to increase the revolution rate of the polygonal mirror, so that it is impossible to avoid fluctuation of the resulting image and it is hence difficult to obtain an image of high quality.